Light pen

ABSTRACT

A light pen for interactive computer graphics consists of an array of, for example, nine photo-electric sensors. Associated logic compares the signals from the various sensors with a common reference signal in order to generate command signals to move a spot of light incident on the array so that it is aligned with the centre of the array.

U nitedjStates Patent 119-1 Kendler et al. 1

15 1 LIGHT-PEN [75] Inventors: Hayden Brian Kendler, llford;

' Lionel George Ripley; David John Woollons, both of Lewes, all ofEngland [73] T Assignee: National Research Development Corporation,London, England 22 Filed; Apr. 19, 1973 21 Appl. No.: 352,692

130] V Foreign Application Priority Data Apr.27, 1972 Great Britain,.19582/7'2 52 us c1.l 250/209, 250/227, 40/324 A 1511 11m. G02b 5 14,G08b 23/00 [58] Field of Seareh'..1..'.... 250/227; 203 CT, 220 114,

250/203 R, 209,217 CR; 340/324 A [56] v Reel-ems Cited UNITED STATESPATENTS 7/1942 Chance at al. 250/220 M 1451 July 23,1974

Primary Examiner-James W. Lawrence Assistant ExaminerT. N. GrigsbyAttorney, Agent, or Firm -Cushman, Darby &

- Cnshrnan [57] ABSTRACT A light pen for interactive computer graphicsconsists of an array of, forexarnple, nine photo-electric sensors.Associated logic compares the signals from the various sensors with acommon reference signal in orderto generate command signals tofmove aspot of light incident on the array so that it is aligned with thecentre of the array.

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NM E2 H u @258 1 2%: L\ X a g mm n5 lull! 4 a g A ON 4 1 LIGHT PEN Thisinvention relates to light pens of the kind used in interactive computergraphics. 1

The use of lightpens in interactive computer graphics is well known. Bypointing such a pen ata cathoderay tube display, an operator is able tocause the computer producing the displayed picture to implement certainpredefined operations such as erasure and'duplication of parts of theimage. Each of these functions requires the operator to be able toindicate to the computer the precise position at which the demandedalteration is to be effected. conventionally, this is achieved using alight pen containing a single photo detector. In

one mode of operation, the controlling computer causes a number of dotsin a defined pattern, such as a cross, to be shown sequentially upon thedisplay screen. After each pattern pointappears, the pen output isinterrogate'd'to determine whether this point has been detected by thesensor. Thus it is possible to establish which of the pattern pointsfalls within the detection field of the pen and, using the knowngeometric properties of the pattern, to compute shift values to be addedtothe component points to centre the pattern beneath the sensor in thedetection head of the pen. If the pen-is moved, the pattern follows ortracks it across the screen and thus can be placed at positions ofinterest. I v I This tracking procedure requires considerableinvolvement of the computer centralprocessorand is extion required toalign it'with the central sensor of the array.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings,'in which: 7

FIG. 1 is a perspective view of a light pen head in accordance with theinvention;

FIG. 2' is an elevational view of the light pen head shown in FIG. 1; V

' FIG. 3 is aschematic diagram ofalight pen assembly in accordance withthe invention;

'FIG..4 is a schematic diagram illustrating part of a cathode ray tubedisplay screen;

' FIG. 5 is another schematic view of a cathode ray tube display screenillustrating the effect of rotating the light pen head;

FIG. 6 is a circuit diagram of a light sensor amplifier forming part ofthe'assernbly shown in FIG. 3; and

the tracking pattern so that the system is capable of tracking a singlepoint on the display screen.

In Gordon A. Rose Light Pen Facilities for Direct View Storage TubesTransactions on Elec-.

tronic Computers, August 1965, page 637, there isdisclosed a light pencomprising a set of four photosensors arranged in horizontal andvertical pairs. Each pair is connected to a respective differenceamplifier which'produces a difference signal indicating horizon-r toprovide a light penwhich is not subject to this limitation.

According to the invention, a light .pen assembly comprises a light penhaving an array of at least three photo-electricsensors and comparator.eans opera tive to determine which of the sensors is receiving thegreatest illumination. I

Preferably the light pen assembly also includes logic means operative inresponse to the comparator means to produce control. signals indicatingthe required movement of the source illumination to bring it to apredetermined position relative to the array of sensors.

Ina preferred embodiment of the invention, the light penhas nine sensorsarrangedin a three by three array and the logic means is arranged toproduce controlsig- FIG. 9 is a schematic diagram of an alternative formof light pen assembly in accordance with the invention.

ends of nine flexible plastic light pipes 1A to l] rigidly set=in amatrix 12- of silicone rubber, epoxy resin or other similar; materialand encased in a thin-walled metal tube 14 of square cross-section. Theexposed ends of the light pipes 1A to 1] are arranged in a 3 X 3 elementarrayfThe other end of each lightpipe is connected to a respectivephoto-detector as will be explained with reference to FIG. 3. Betweenthe tube 10 and the unit (not shown) housing the photo-detectors, thelight pipes are gathered together and sheathed in protective plasticsleeving 16;

Referring to FIG. 3, each of the light pipes 1A to U is connected to arespective photo-detector 3A to 3J. The outputs=of the photo-detectors3A. to SJ are connected'to respective variable gain amplifiersAA to 4]which are used to compensatefor differences in sensitivity between thevarious sensor channels. The outputs .ceive equal illumination, only thecomparator in the sensor channel producing the greatest signal can producea logical "1 output, all of the others producing a logical 0. If twoor more of the light pipes are equally illuminated so as to be producingthe greatest signal output, the corresponding comparators will eachproduce logical l outputs. The outputs of the comparators 5A to SJ areconnected to respective bistable latches 6A to 6], the reset inputs ofwhich are connected via lead 19 to receive a resetting signal coincidentwith the bright-up strobe of the tracking spot so that the comparatoroutput is sampled during each field period of the cathode raytubedisplay immediately 3 after display of the tracking spot. The outputs ofthe bistable latches 6A to 6] are connected to a coding logic unit 20,the function of which will be explained hereinafter.

FIG. 4 shows part of a cathode ray tube screen 22 confronted by thelight pen, the areas 7A to 7] being confronted by the exposed ends ofthe light pipes 1A to U respectively. It should be understood that theboundaries of these areas are not marked on the screen in any way andare shown in the drawing for convenience of representation only. Thus,when the tracking spot of the display is in its required positionconfronting the light pen, it occupies the area 7] on the displayscreen. In this condition, comparators A to SJ will determine that thephoto-detector 3.] is producing the largest output and consequently thelatch 61 will be set.

In operation, the light pen head 10 is positioned so that at least oneof theexposed ends of the light pipes 1A to U is illuminated by thetracking spot. The coding logic unit sends commands to the computerindicating the required movement of the tracking spot to bring it intoalignment with the end of the central light pipe 1]. Thus, for example,if initially the spot is located in the area 7C so that it illuminatesthe light pipe 1C, the

required movement of the spot is downwards and to the left. The requiredmovement of the spot for each of the nine areas is indicated in table 1where zero indicates no movement and one indicates movement by a singleincrement.

Movement Required Area of- Illuminated The increment size which isusedis preferably slightly less than the spacing between the centres of theexposedv ends off-adjacent light pipes. With this spacing, centririg ofthe spot in general occupies only a single iteration. Larger incrementsmay cause oscillation of the spot or even may cause it to escape fromthe detection field. Smaller increments increase the time taken tocentre the spot since several iterations may be required.

lngeneral, if more than one light pipe receives maximum illumination sothat more than one of the latches 6A to 6J is set, the appropriatecommand will be produced. For example, if the spot centre is located atthe point 24 in FIG. 4, maximum illumination will be simultaneouslyreceived by the photo-detectors 3A, 3B, 3H and 31. This will cause thespot to be moved down and to the right so that its centre is located atpoint 26 in FIG. 4. After the next scan, if the pen head 10 remainsstationary, the spot centre will be moved back to the point 24. However,this cycle is harmless.

Excessive spot brightness or defocussing might lead to simultaneousillumination of, for example, photodetectors 3A, 3G and 3H, thusproducing commands up, down and right. In order to prevent this, therequirement for an up command is arranged to inhibit the production of adown command and vice versa. Thus, this situation would result in theproduction of a command to move the spot to the right. Thus, the logicprovided by the coding logic unit 20 is as follows:

UP=(E+F+G)- m DOWN (A B Q m LEFT= c D E)' m RIGHT (A o H) (e rors) Thecoding logic unit 20 provides logical'l on output lead 28 when an UPmovement is required, on 30 when a DOWN movement is required, on 32 whena LEFT movement is required and on 34 when a RIGHT movement is required.In addition, the coding logic unit 20 has a fifth output lead 36.Innormal operation, logical 1 is produced on this output lead whenmaximum illumination is received by the central light pipe 1]. This canbe usedto provide a signal to the computer to indicate that the trackingspot is properly'centred with respect to the light pen head. Inaddition, by using this output alone, the apparatus may be used as aconventional light pen. Alternatively, if a light pen with a largedetection field and correspondingly low resolution is required, theoutputs from all nine photo-detectors 3A to 3J may be combined and usedto provide logical l on the output lead 36 if the cathode ray tube spotis detected by any of the photo-detectors 3A to 3].

FIG. 5 illustrates the effect of rotating the light pen head through.45.If the centre of the tracking spot'is initially at position 38 inthearea 7A on the cathode ray tube screen, it will be moved to the rightand down so that at the end of the first iteration, it is at position 40in area 7B. In the next iteration, it is moved down to 'position 42 inarea 7D and in a third iteration, to the left so that its final positionis position 44 in area 71. Thus, what would take one iteration if thepen 10 was correctly oriented has taken three iterations with the penrotated through 45 but the tracking spot has nevertheless been correctlypositioned within the required area 7.] A similar result is obtained ifthe tracking spot is initially in any of the other outer areas 7A to 7H.In practice, the inherent rigidity provided by the light pipes 1A to I]gathered together in their plastic sheath 16 is sufficient to makeaccidental rotation of the light pen by as much as 45 highly unlikely.

Referring to FIG. 6, each of the photo-detectors 3A 1 to 3] consists ofa photosensitive field effect transistor 46 and an associated amplifierconsisting of a first common-emitter stage comprising a bipolartransistor 48 and a final emitter follower stage employing a similartransistor 50. The field effect transistor 46 is optically coupled toreceive illuminations from its associated light pipe as indicated by thearrow 52. Transitory il- Iumination of the field effect transistor 46due to passage of the cathode ray tube spot past the sensor headgenerates extra carriers at the gate-channel junction. This causes anincrease in the reverse gate leakage current and a consequent rise ingate potential. A similar rise in source potential occurs due to thesource follower configuration in which the field effect transistor isconnected. A very high sensitivity and noise performance is obtainedbecause of the very high resistance biasing networks which can be used.The positive voltage pulse produced by the field effect transistor 46 isamplified by the transistor 48 and a negative output voltage .8A to 8]to a 0.15/11; capacitor 56 which becomes charged within 0.7 volts of thegreatest signal level. A high input impedance source follower circuitincluding a field effect transistor 58 monitors the voltage on thecapacitor 56 and drives a compound emitter follower stage comprisingtransistors 60 and 62 which supplies the reference voltage to thecomparators 6A to .6] (FIG. 3). The connection between the field effecttransistor 58 and the transistor 60 is via a potentiometer 64 which isused to provide a constant D.C. offset of 0.5 volts between the inputsto the diodes 8A to SJ and the output to 62. This insures that there isan adequate difference between the inputs of the comparatorin thechannel receiving the highestillumination to ensure reliable setting ofthe associated latch 6. i

The coding logic unit 20 is illustrated in FIG. 8. In order to providethe UP and DOWN commands, the three channels E, F and G requiring an UPcommand when the tracking spot is detected bythe correspondingphoto-detector are connected to a first OR gate.70 and the threechannels requiring a DOWN command are connected to a second OR gate 71.The output of the OR gate 70 is connected directly to an AND gate 72,which provides the UP command on lead 28, and via an inverter 73 to anAND gate 74, which provides the DOWN command on lead 30. Thusillumination of any of the photo-detectors 3A, 3B and 3C inhibitsproduction of an UP command even if one of the photo- 'detect'ors 3E, 3For 3G is also illuminated and illumination of any of the photo-detectors3E, SP or 36 inhibits production of a DOWN command evenif any of thephoto-detectors 3A, 3B and 3C is also illuminated.

A similar arrangement, comprising OR gates 80 and 81, AND gates 82 and84 and inverters 83 and 85 controls the production of LEFT and RIGHTcommands.

I To produce the signal on lead 3G indicating that the spot is centred,the I channel is connected directly to a five input AND gate 76, theother four inputs of which are connected, via respective inverters 77,78 79 and 80 to the outputs of the AND gates 72, 74, 82 and 84.

FIG. 9 illustrates an alternative embodiment of the invention in whichthe peak level detection circuit 18 is omitted. Instead, the secondinput of each of the comparators A to 51 is connected to lead 90 towhich a fixed reference voltage is applied. This reference voltage ischosen to havea magnitude higher than that of the output from anyphoto-detector due to ambient i1- lumination. Thus, if the spot issymmetrically disposed on the division between the two sensors, both ofthe corresponding comparators will produce logical l outputs. If thespot size is larger than the end of a light pipe, a' greater numberof-the comparators SA-to SJ may produce logical 1 output. As alreadyexplained,

the coding logic -is designed to prevent contradictory commandsappearing on the leads 28 to 34 in the event that more than one of thecomparators 5A to 5] is producing a logical 1 output.

Instead of using a single reference voltage on lead 70, a separatereference. voltage may be provided for each of the comparators 5A to 5].Since these reference voltages can be individuallyadjustedto compensatefor differences in sensitivity between the channels, the variablegain'amplifiers 4A to M can be omitted. Each of the photo-detectors 2Ato. 21, with its associated comparator, now forms a variable thresholdlight operated switch and can be replaced by a composite deviceperforming this function.

The manner in which a computer can be programmed to respond to theoutputs on leads 28 to 36 to control the position of a tracking spot ona cathode ray tube display is well known.

We claim: i 1

1'. A' light pen assembly having a two-dimensional array of at leastthree photo electric sensors andcontrol means for comparing the outputof each sensor with a common reference signal and logic means operativein response to the control means to produce control signals indicatingthe required movement of illumination to bring it to a pre-determinedposition relative to the array of sensors.

. 2. A light pen assembly as claimed in claim I, in which the arraycomprises at least four photo-electric sensors, one of said sensorsbeing disposed at the centre of the array.

3.-A lightpen assembly as claimed in claim 1, in which eachphotoelectric sensor comprises a light pipe having a photocell at oneend thereof, the ends of the various light pipes remote from thephotocells being grouped to form the array.

4. A light pen assembly as claimed in claim 2 in which thelevel of thereference signal is dependent on the output from the photo-electricsensor which is receiving the greatest illumination.

'5. A light pen assembly as claimed in claim 4, in which the controlmeans includes a peak level circuit comprising a respective diode foreach sensor connecting the sensors to a common circuit point and havingpolarity such that the diodes connected to all sensors having loweroutput levels than the output level of the sensor receiving the greatestillumination are reversed biased, and means for providing a referencesignal at a level higher by a predetermined amount than the highestlevel output from the diodes.

6..A light pen'assembly as claimed in claim I, in which the referencesignal is at a predetermined threshold level and the logic means isoperative in response to those of the photo-electric sensors havingoutputs above said reference level to produce said control signals andincludes means for inhibiting both of any such pairs of control signalsindicating motion in opposite directions.

1. A light pen assembly having a two-dimensional array of at least threephoto-electric sensors and control means for comparing the output ofeach sensor with a common reference signal and logic means operative inresponse to the control means to produce control signals indicating therequired movement of illumination to bring it to a pre-determinedposition relative to the array of sensors.
 2. A light pen assembly asclaimed in claim 1, in which the array comprises at least fourphoto-electric sensors, one of said sensors being disposed at the centreof the array.
 3. A light pen assembly as claimed in claim 1, in whicheach photo-electric sensor comprises a light pipe having a photocell atone end thereof, the ends of the various light pipes remote from thephotocells being grouped to form the array.
 4. A light pen assembly asclaimed in claim 2 in which the level of the reference signal isdependent on the output from the photo-electric sensor Which isreceiving the greatest illumination.
 5. A light pen assembly as claimedin claim 4, in which the control means includes a peak level circuitcomprising a respective diode for each sensor connecting the sensors toa common circuit point and having polarity such that the diodesconnected to all sensors having lower output levels than the outputlevel of the sensor receiving the greatest illumination are reversedbiased, and means for providing a reference signal at a level higher bya predetermined amount than the highest level output from the diodes. 6.A light pen assembly as claimed in claim 1, in which the referencesignal is at a predetermined threshold level and the logic means isoperative in response to those of the photo-electric sensors havingoutputs above said reference level to produce said control signals andincludes means for inhibiting both of any such pairs of control signalsindicating motion in opposite directions.